Adjustable coupling system



L. F. CURTIS ADJUSTABLE COUPLING SYSTEM Filed Dec. 5l, 1942 March 19, 1946.

Patented Mar. l19',v 194.6`

ADJUSTABLE coUrLlNG SYSTEM Leslie F. curtis',Gi-eat Neck, N. Y., assigner te Hazeltine Corporation, a corporation of Dela- Application December 31, 1942, Serial No. 470,773

Claims.

The present invention relates to adjustable coupling systems and, particularly, to such systems ot the type which are adapted to adjust the magnitude and polarity of coupling between at least two inductors.

Two inductors which are inductively coupled are said to have mutual inductance. Where the inductors are aperiodic or nonfrequency-selective, the voltage induced in one usually has a phase angle of approximately 90 degrees with respect to the current in the other. It is frequently desirable in such aperiodic inductive-coupling systems that the voltage induced in one inductor shall bein the sameor opposite phase with the current in the other inductor and that thisphase relationship shall not appreciably change with adjustment or" the amount of coupling between the inductors over a predetermined range. Additionally, in certain applications it is desirable that the induced voltage shall be proportional to the square of the operating frequency. Where the induced voltage is in the same or opposite phase with the applied current as previously described, the inductive-coupling system may be said to have no reactive component of coupling but only a resistive component of coupling. That is, the system operates as though the coupling were effected by the use of a resistor. It is desirable that this resistance coupling of the system shall be controllable in magnitude without the use of any sliding contacts or iiexible connections and it is frequently quite deinductors and in another position in its path of sirable thatA it shall be controllable from an eiTec- Y tive Y itive resistance through zero to an effectiv egative resistance.

It is an object of the present invention, there-z fore, to provide a new and improved adjustable coupling system of the type described which possesses one or more of the desirable features hereinbefore specied.

It is a further object of the invention to provide a new and improved adjustable coupling system adapted continuously to adjust the magnimotion it is substantially coupled with yboth of the inductors, whereby the coupling means provides primarily resistance coupling between the inductors. disposed in the magnetic eld of the first inductor that it has a predetermined minimum coupling with thefirst inductor for at least one position of the inductive-coupling means. The coupling system also includes 'means for moving the coupling means over at least a portion of its path of motion to adjust the value of the resistance coupling between the inductors.

In a particular form of the invention, the rst inductor has Atwo spaced portions connected to have opposing iields and the second inductor is positioned between these portions. The movable inductive-coupling means preferably has a ratio of resistance to reactance of the order of ten and preferably comprises a coupling cylinder wherein the desired high'ratio of resistance to reactance tude, between positive and negative limits, of a Y resistive component of coupling between two inductors.

It'is an additional object of the invention to provide an improved adjustable coupling system which is aperiodic or nonselective over a' wide range of operating frequencies and which provides mainly resistive coupling between inductors of magnitude proportional to the square of the operating frequency.

It is a further object of the invention to provide an improved adjustable coupling system for continuously adjusting the magnitude and polarity is eiected by a relatively thin cylinder wall.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 represents a cross-sectional view of an adjustable coupling system embodying the present invention; Fig. 2 illustrates a fragmentary portion" of the Fig. 1 arrangement and shows the disposition and electrical connections of the inductors thereof; Fig. 3 is a graph representing the magnitude and polarity of coupling eifected within the range of adjustment of the Fig. 1 coupling system; Fig. 4 represents a simplified equivalent circuit diagram of the Fig. 1 coupling system and is used as an aid in explaining the operation of the invention; Fig. 5 is a circuit diagram, partly schematic, of a complete electrical indicating system which embodies The second inductorl is so wound and the adjustable coupling system of thepresent invention; Fig. 6 illustrates a modified form of coupling cylinder suitable for use in the coupling system ofthe invention; Fig. '7 represents a modified form of the invention; and Fig. 8 is a graph representing the magnitude and polarity of resistance coupling effected by the Fig. '7 modification.

Referring now more particularly to Fig. 1 of the drawing, there is`represented a complete adjustable coupling system embodying the present invention in a preferred form. The system includes a first inductor I having two spaced windings or portions Illa and Illb which are wound in suitable grooves II and I2, respectively, provided on the periphery of a cylindrical member or form I3 of insulating material. The coupling system also includes a second inductor. I4 which is wound in an intermediate groove I5 of the member I3 and hence is coaxial with'the first inductor I0 and centrally positioned between the portions I0 and I0 thereof. There is also provided in the coupling system an axially movable conductive nonmagnetic inductive-coupling means having a value of resistance much greater than its value of reactance over an operating frequency range and being aperiodic and so disposed with reference to the'inductors I il and I4 that in a given position in its range of motion it is substantially uncoupled with at least one of the inductors and in another position in its range of motion it is substantially coupled with both of the inductors, whereby the coupling means provides primarily resistance coupling between the inductors. This inductive-coupling means, which preferably has a ratio of resistance to reactance of the order of ten, comprises a coupling cylinder I6 of conductive material and the desired high ratio of its resistance to its reactance is effected by constructthe same direction on the insulation member I3.-

One end terminal of the portion I0.. is connected to the corresponding end terminal of the portion IIlb and the other terminals of these portions are designated in Fig. 2 as a and b, respectively, and

constitute the terminals of the inductor I0. Magnetic fields thus produced by the inductor portions llla and Ib have opposing relation. The in-Y ductor portions los and Illb preferably have the same number of turns, whereby the position of zero magnetic field intensity occurs midway between them. The second inductor I4, in the position shown, is midway between the inductor portions IIla and Illb and hence is so disposed in the magnetic eld of the first inductor I0 that the second inductor I4 is substantially uncoupled with the first inductor.

Considering now the operation of the adjustable coupling system, assume that the inductor l0 is excited by an alternating current. The opposed electromagnetic fields produced Iby the inductor portions Illa and IIlb induce corresponding but opposite currents in the coupling cylinder I6, these ing it in the form of a relatively thin cylinder, the

ratio of the cylinder wall thickness to cylinder external diameter being not substantially greater than one-seventh. The coupling cylinder I6 may be formed of any conductive nonmagnetic material, but the thickness of its cylinder wall should vary inversely with the specific conductivity of the material. Thus, if the coupling cylinder I6 is formed of brass, its wall may be relatively thick compared to that of a coupling cylinderv formed of copper or aluminum.

The adjustable coupling system also includes means for moving the coupling means or cylinder I6 at least over a portion of its range of motion to adjust the value of the resistance coupling between the inductors. 'Ihis means comprises a screw-threaded member Il which is threaded through a flange I8, the latter having on its inner face a groove I9 in which the insulating member I3 is secured at one end. The flange I8 is provided with apertures 20 through which machine screws or the like may be inserted to secure the coupling system to a panel or other support. The member to the screw-threaded member I1 for rotating the latter to move the coupling cylinder I6 with respect to the induetors III and I4.

The two portions- Illa and I0 of the inductor I0 are connected and excited to have opposing magnetic fields. One manner in which this may/b done is seen by reference to Fig. 2 which illuscurrents at any given instant flowing around the periphery of the cylinder but in opposite directions. Assume further that the coupling cylinder I6 is positioned equidistant from the inductor po-rtions Illa and Illb and that the magnetic fields produced by the latter are equal, as would occur if the inductor portions IIla and Ib had equal numbers of turns. The currents induced by these fields in the coupling cylinder I6 are then equal and the resultant fields which they produce are likewise equal but opposing and consequently are ineffective to induce any voltage in the centrally positioned inductor I4. It will be observed that inductor I4 is so Wound and disposed in the magnetic field of inductor I0 that it has a predetermined minimum coupling with the inductor I4 for at least one position of the coupling means II.V In this embodiment, the minimum coupling is substantially zero when coupling means I6 is in a central position within inductor I4.

Assume now that the knob 24 is rotated to move the coupling cylinder I6 axially to a position where it is closer to the inductor portion Illa than to the inductor portion I 0b. The coupling cylinder I6 is now more closely coupled to the inductor portion |03 and the magnetic field produced by current flowing through the latter induces a current in the coupling cylinder I6 which is larger than that induced bythe field of the inductor portion it. The fieldsproduced by these unequal induced currents are correspondingly unthe largest induced current in coupling cylinder I6. Under the conditions assumed, the induced potential in inductor I4 is approximately in phase with the current in the inductor portion I0.. This potential has a magnitude which increases asthe coupling cylinder I6 is moved more closely to the inductor portion I0a.

If now the coupling l ol the voltage induced inrent owing in the inductor I 0, and thus the coucoupling cylinder I4 which, under the conditions last assumed, is

approximately in phase with the current ilowing in the inductor portion Ib; As before, the magnitude of this induced voltage varies with the closeness of spacing of the coupling cylinder I6 to the inductor portion IIlb. That is, the coupling cylinder I6 eilects a coupling between the inductor I4 and one of the inductor portions I 0., l0.

'Ihe manner in which the magnitude and phase the inductor I 4 by curpling between inductors III and I4, varies with movement of the coupling cylinder I6 is represented by the graph of Fig. 3. It will be seen from this iigure that the induced voltage'and coupling are zero when the coupling cylinder is centered with relation to the inductors I0 and I4 and that it has a given phase and a magnitude which increases with movement of the coupling cylinder in either direction from center. 'From this it will be seen that the adjustable coupling system of the invention is eiective to provide primarily resistance coupling between the inductors I and I4 and that this resistance coupling may be either positive or negative; -that is, it is equivaient to coupling produced by a resistor' selectively having either a positive resistance or eiTectively a negative resistance.

cylinder Is due to this induced voltage is determined by the relation:

Ria -i-jwLls (2) The voltage induced in the inductor I4 by the resultant current owing in the coupling cylinder It may be stated at this point that the magnitude of the voltage induced in the inductor I 4 varies not only with the relative position of the coupling cylinder I6, as previously described, but varies also with the frequency of the current owing in the inductor I0. Speciiically, the induced voltage varies as the square of the operating frequency. This will be more readily apparent from the following mathematical analysis of the operation of the coup 'ng system which considers the I6 as equivalent to a small lnductor and a large resistor serially connected in a closed circuit. Referring to Fig. 4, which represents a simpliiied equivalent circuit diagram of the coupling system of the present invention, the inductor Lio represents the inductor I0 lwhich comprises the inductor inductor L14 represents the inductor I 4, and the inductor L16 and resistor Rie represent an inductor and resistor equivalent to the corresponding constants of coupling cylinder I6. The resultant mutual inductance between the inductor I0 and the coupling cylinder AII; is indicated in Fig. 4 as Mio-1e. The' mutual inductance between the cylinder I6 and the inductor I4 is represented in Fig. 4 as Mid-14. It will be understood that no mutual inductance exists between inductors I0 and I 4.

It can be shown that the voltage induced in the coupling cylinder I6 by current yiiowing in the inductor I0 has the following value:

where The resultant current which ows in the coupling portions Lion and Liob, the

1 quency range. A

I6 is determined by the relation:

I M 'M EN: uw io 1s 14s-i4 RMHQLM (A3) Now since:

M10-is klo-IW LioLie (4) where Y k1o-1s=the coefficient of coupling between inductors Lw and Lis k1s-14=the coeicient of coupling between inductors L14 and Lis Equation 3 may be written in the form:

'tion 6 simpliiies into the form:

Equation 7 may also be Written in the form;

(8) where 1 Qiev=the ratio of inductive reactance to resistance tion 7 that the voltage induced in the inductorr I4 varies with the square of the frequency of the current owing in the inductor I0, as previously stated. It is apparentfrom Equation 6 that the resistance of the coupling cylinder I6 should be made large With respect to its inductive reactance, these circuit parameters appearing in the denominator of Equation 6 and being quadraturerelated impedance components. l

Fig. 5 is a circuit diagram, partly schematic,-of a complete electrical indicating system andf'represents a type of arrangement ln which. the, adjustable coupling system of the present-invention has particular utility. An indicating system of this general type is. disclosed and claimed in a copending application of Harold A Wheeler, Serial No. 470,786, filed December 31, 1942. A system of this type may be used, for example, to locate bodies of ore or metallic objects buried below the surface of the ground. This indicating system includes an amplifier 25 having inputcircuit terminals 26 and output-circuit terminals 21 and having substantial ampliiication and a positive phase slope over a predetermined-fresingle feed-back path is-provided for coupling the output-circuit terminals 21 to the input-circuit terminals 26, this feedtransformer 28 in a primary circuit which is coupled to the output-circuit terminals 21 of the amplifier 25. 'A

The exploring unit 29 also includes a third inductor 33 having a diameter intermediate to that of the inductors 30 and 3l. The inductor 33 vis included in series with the secondary winding 34 of transformer 28 in a secondary circuit which is coupled to the input-circuit terminals 26 of the amplifier 25. The inductor 33 of exploring unit 29 is so positioned with relation to the inductors 30 and 3| that it normally is uncoupled to the latter and the total feed-back energy from the output-circuit terminals 21 to the input-circuit terminals 26 is thus provided primarily by the transformer 28. The component of feed-back couplingprovided by the latter transformer is primarily reactive in nature; that is, the voltage induced'in the secondary winding 34 of transformer 28 has a quadrature-phase relation .with respect to the current owing in the primary winding 32. y

Due to metallic structural parts necessarily employed in the construction of the exploring unit 29, an undesired though small resistive component of feed-back coupling occurs in the unit 29. To compensate for this undesired feedback, the adjustable coupling system 35 of the present invention is included in the' feed-back path of the amplifier 25. The first inductor II) of the coupling system is serially connected in the primary circuit, which is coupled to the output-circuit .terminals 21 of the amplifier 25, while the second inductor I4 of the coupling lsystem is serially connected in the secondary circuit, which is coupled to the input-circuit terminals 28 of amplifier 25.

Considering briey the operation of this indicating system, the feed-back energy provided by the transformer 28 causes the amplier 25 to oscillate, the frequency `of oscillation being such that the total phase shift through the amplifier and feed-back path is zero. If now a metallic body isbrought into the-vicinity of the exploring unit 29, a component of feed-back energy in v`|uadrature relation with that provided by the transformer 28 occurs in the exploring unit 29 and has the effect of changing the phase shift of oscillations translated through the feed-back path. This results in a shift of the frequency of .i oscillation of amplifier 25 with the result that a beat-note signal is produced between the oscillations of amplifier 25 and a beat-note oscillator included therein. The beat-note signal is applied to an indicating device 36, thereby to indicate a change in the frequency of oscillation of the am I plifier 25 thus indicating the presence of the metallic body in the vicinity of the exploring unit 29.

Since metallic structural members used inthe construction of the exploring unit 2 9 produce the same type of coupling between the windings of exploring unit 29- as does a metallic object brought intothe vicinity thereof, the system is initially balanced to compensate for the effect of such metallic structural members by suitable adjustment of the coupling system 35. This is effected by so adjusting the coupling cylinder I6' of the system 35 with-relation to the inductors I0 and I4 thereof that a resistance component lof 'coupling of the proper magnitude, and of phase opposing that introduced by the structural members of the exploring unit 29. is fed back from the output-circuit terminals 2.1 through the coupling system 35 to the input-circuit terminals 29 of amplifier `25. The resistance component of coupling provided by the coupling system 35 is thus just equal to but opposes that introduced by structural members of exploring unit 29 and hence the undesired effect of the'latter members is compensated.

Fig. 6 represents a modified form of. coupling cylinder which may be used in place of the coupling cylinder I6 of the Fig. 1 arrangement. `The coupling cylinder I6 in Fig. 6 is comprised of a closed ring or cylinder having a large segment 31 of relatively large cross-sectional area and of a conductive material exhibiting relatively high electrical conductivity in order that this segment shall have a very low ratio of inductive reactance to resistance. Suitable slots 38 and 39 are provided in the ends of the segment 31 and there is secured in the slots, 'in conductive relation with the segment 31, a smaller segment 40 of relatively'small :cross-sectional area and formed of a conductive material exhibiting lower electrical conductivity or high resistance, for example nickel or Nichrome. In a particular application, the segment 31 was formed of copper bar having a one-eighth inch square cross section and bent into a three-quarter inch diameter ring, the segment 40 being formed of a sheet of nickel a few thousandths of an inch thick, one- .eighth inch Wide and one-eighth inch long.

This form of conductive and non-magnetic in- -'ductive-coupling means has a value of resistance much greater than its value of reactance over the operating frequency range of the coupling system. In operation, a coupling system using a coupling means embodying the Fig. 6 construction operates in the same manner as that of the Fig. 1 arrangement and, hence, the. de, scription thereof will not be repeated.

In certain applications, it may be desirable given phase is inducedin the inductor I4 by current flowing in the inductor I0 when the coupling cylinder I6 is centered within the inductor I4 and that this induced voltage becomes zero only when the coupling member I6 is moved to a position where it is much closer to the inductor portion Ib than to the inductor portion Illa. It will be observed that inductor v I4 has a predetermined minimum couplingwith inductor I0 when cylinder I8 is in this last-mentioned position. A reversal of phase of the voltage induced in the inductor I4 is effected when the coupling cylinder I6 is moved quite closeto the inductor portion Illb, but the magnitude of the induced voltage is not so large as when the coupling cylinder I 6 is moved into close proximity with theinductor portion lui..`

While the operation of the invention has'been explained hereinbefore on the assumption that v vand i4' regardless of which inductor is the priy mary inductor.

Asillustrative of a specific embodiment of the invention, the following parameters are given for an embodiment of the invention of the type shown in Fig. 1:

Insulating member I3: l 5

Inductor Vportions i;

and Ill each 9 turns of No. 26

. single silk cove r e d enameled moving said coupling means over at least a pori wire wound in i two layers. 'i Inductor Il 9| turns of No. 26

single silk covered enameled wire wound in two layers. Coupling cylinder-AIS, of brass:

Outside diameter inch 0.360 Inside diameter ,do 0.308 Length do While it has hereinbefore been stated that the movable inductive-coupling means, comprising the cylinder IS for example, is lso disposed with reference to the inductors I0 and I4 that in a given position in its Ipath of motion it is substantially uncoupled with at least one of the inductors and in another positionin its'path of motion it is r`will be apparent that an adjustable coupling system embodying the invention has the advantages that it provides a continuous adjustment, between positive and negative limits, of the magnitude of a resistive component of coupling between two inductors. It has the additional adand modications as fall within the true spirit and scope o! the invention.

What is claimed is: J

, 1,'An adjustable coupling systemladapted to operate over a given frequency range comprising, a first inductor, a second inductor, a movable conductive nonmagnetic inductive-coupling means having a value of resistance greater than its ,value of reactance over the operating frequency range and being aperiodic and so disposed with reference to said inductors that in a given position in its path of motion it is substantially uncoupled with' at least one of said inductors and invanother position in its path of motion it is substantially coupled with both. of said inductors, whereby said coupling means .provides primarily resistance coupling between said inductors, said second inductor being so wound and disposed in the magnetic iieid of said rst inductor that it has a predetermined .minimum coupling with said iirst inductor for at least one position of said coupling means, and means for tion of said path to' adjust thevalue of said resistance coupling between said inductors.

2. An adjustable coupling system adapted to operate over a given frequency range comprising, a first inductor, a second inductor, a mov- A able conductive nonmagnetic inductive-coupling means having a value of resistance greater than its value of reactance over the operating frequency range and being aperiodic and so disposed with reference to said inductors that in a given` position in its path of motion'it is substantially uncoupled with at least one of'said inductors and in other positions in its path oi motion on opposite sides of said given position it substantially couples said inductors with a polarity dependent upon the directionl in said path of motion from said given position, whereby said coupling means providesv primarily resistance coupling between said inductors, said second inductor being so wound and disposed in the magnetic field ofsaid iirst inductor that it has a predetermined minimum coupling with said first inductor for at least one position of said coupling means, and means vantage that it is aperiodic or nonselective overv a awide range of operating frequencies but provides mainly la resistive coupling between its inductors of magnitude proportional to the square of the operating lfrequency. There is the additional advantage that the polarity of the resistive coupling may be selected as'desired and the magnitude of the couplingmay becontinuously addusted in an arrangement which avoids the `use of any sliding contacts or ilexlble connectors.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes for moving said coupling means over at least a portion of said path to adjust the value of said resistance coupling between said inductors.

3. An adjustable coupling system adapted to operate over a given frequency range comprising, a iirst, inductor, a second inductor, a movable conductive nonmagnetic inductive-coupling means having a value of resistance greater than its value of reactance over the operating frequency range and being aperiodic and so disposed with reference to said inductors that in a given position inits path ofv motion it is substantially uncoupled with one of said inductors and lsubstantially coupled with the other o1' said inductors and in another position in its path of motion it is substantially coupled with both of said inductors, whereby said coupling means provides 4primarily resistane coupling between said induc- Iof said path' toadjust the value of said resistance coupling between said inductors.

4. An adjustable-coupling System adapted to operate .over a giventfrequencyrange comprising, a rst inductor, a second inductor, a movable conductive non magnetic inductive=coupling means having a ratio of resistance to inductive reactance of the order of ten over the operating frequency range and being aperiodic and so disposed with reference to said inductors that in a given position in its path of motion it is substantially uncoupled with at least one or said inductors and in another position in its path of motion it is substantially coupledwith both oi said inductors, whereby said coupling means provides primarily resistance coupling between said inductors, said second inductor being so wound and disposed in the magnetic held of said first inductor that it has a predetermined minimum coupling With said first inductor for at least one position of said coupling means, and means for moving said coupling means over at least a portion of said path to adjust the value of said resistance coupling between said inductors.

5. An adjustable coupling system adapted to operate over a given frequency range comprising, a rst inductor, a second inductor substantially coaxial with said rst inductor, an axially movable conductive nonmagnetic inductive-coupling means having a value of resistance greater than its value of reactance over the operating fre-v quency range and being aperiodic and so disposed with reference;y to said inductors that in a given position in its path of motion it is substantially uncoupled with atleast one of said linductors and in another position in its path of motion it 'is substantially coupled with both of said inductors, whereby said coupling means kprovides primarily resistance coupling between said inductors, said vsecond inductor being so wound and disposed in the magnetic held of said l iirst inductor that it has a predetermined minimum coupling with said rst inductor for at least one position of said coupling means, andmeans for moving said coupling means over at least a portion of said path to adjust the value of said resistance coupling between said inductors.

6. An adjustable coupling system adapted to operate over a given frequency range comprising, a nrst inductor, a second inductor, a movable `conductive nonmagnetic inductive-coupling cylinder having a value of resistance greater than its value of reactance over the operating rrequency range and being aperiodic and so dis= posed with reference to said inductors that in a given position in its path of motion it is substantially uncoupled with at least one of said inductors and in another position in its path of motion it is substantially coupled withboth of said inductors, whereby said colviplingv cylinder provides primarily resistance coupling between said inductors, said second inductor being so wound and disposed in the magnetic eld of said hrst inductor that it has a predetermined mini` mum coupling with said rst inductor ijor at least one position of said coupling cylinder, and means for moving said coupling cylinder over at least a portionl of said path to adjust the value of said resistance coupling between said inductors.

7. An adjustable coupling system adapted to operate over a given frequency range comprising, a rst inductor7 a second inductor, a thin-walled movable conductive nonmagnetic inductive-coupling cylinder having over the operating frequency range a value of resistance greater than least one of said inductors and in another position in its path oi. motion it is substantially coupled with both of said inductors, whereby said coupling cylinder provides primarily resistance coupling between said inductors, said second inductor being so wound and disposed in the magnetic held of said first inductor that it has a predetermined minimum coupling with said first inductor for at least one. position of said coupling cylinder, and means -for moving said coupling cylinder over at least a portion of said path to adjust the value of said resistance coupling between said inductors. l

8. An adjustable coupling system adapted to operate over a given frequency range comprising,

a irst inductor having two spaced portions connected to have opposing magnetic elds, a second inductor positioned between the portions of said rst inductor, a movable conductive nonrnagnetic inductive-coupling means having a value of resistance greater than its value of reactance overthe operating frequency range and being aperiodic and so disposed with reference to said inductors that in a given position in its path of motion it is substantially uncoupled with at least one of said inductors and in another posil tion in its path of motion it is substantially couwhereby said coupling means provides primarily pled with both of said inductors, whereby said coupling means provides primarily resistance coupling between said inductors, said second inductor being so disposed in the magnetic eld of said :first inductor that it has a predetermined minimumcoupling with said rst inductor for at least one position of said coupling means, and means for moving said coupling means over at least-a portion of said path to adjust the value of said resistance coupling between said inductors.

9.|An adjustable coupling system. adapted-to operate over a given frequency range comprising, a :first inductor having two spaced portions connected to have opposingmagnetic elds, a second inductor positioned between the portions of said rst inductor but closer to one portion than to the other, a movable conducive nonmagnetic inductive-coupling means having a value of resistance greater than its value of reactance over the operating frequency range and beinglaperlodic and *so disposed with reference to said inductors that in a given position in its path of motion it is substantially uncoupled with at least one ofsaid inductors and in other positions in its path of motion it is coupled with both of said inductors,

resistance coupling between said inductors, said second inductor being so disposed in the magnetic eld of said lrst inductor that it has a predetermined minimum coupling with said rst inductor for at leastone position of said coupling means, and means for moving said coupling means over at least a portion of said path to adjust the value of said resistance coupling between said inductors.

10. An adjustable coupling system adapted to operate over a given frequency range comprising, a first inductor, a second inductor, a movable conductive nonmagnetic inductive-coupling means having over the operating frequency range a value of resistance greater than its value of reactance its value of reactance, said coupling cylinder being aperiodic and so disposed with reference to said inductors that in a given position in its path of motion it is substantially uncoupled with at and comprising a closed ringhaving a large segment of relatively large cross-sectional area and of a material exhibiting high electrical conductivity and a smaller segment of relatively small cross-sectional area and of a material exhibiting lower electrical conductivity, said coupling means being aperiodic and so disposed with reference to said inductors that in a given position in its path of motion it is substantially uncoupled with at least one oi' said inductors and in another position y in its path of motion it is substantially coupled with both of said inductors, whereby said coupling means provides primarily resistance couplinc between said inductors, said second inductor bein: so wound and disposed in the magnetic tleld oi' said nrst Inductor that it has a predetermined minimum coupling with said rst Inductor for at least one position of said coupling means, and

means for moving said coupling means over at least a portion of said path to' adjust the value of saidv resistance coupling between said inductors.

f LESLIE F. CURTIS. 

